This invention relates to oil base well drilling and servicing fluids. In particular, the invention relates to “all-oil” and “invert oil” emulsion well drilling and servicing fluids containing an oil-soluble polymeric fluid loss control additive solubilized therein.
As is well known in the art, invert emulsion oil based well drilling and servicing fluids, generally called “muds”, are water-in-oil emulsions that typically contain an organophilic clay viscosifier/suspension additive, and a weighting agent. The water phase is usually a solution of a salt, such as calcium chloride or sodium chloride, whose concentration is normally adjusted such that the aqueous activity of the fluid is equal to or less than the aqueous activity of the subterranean formations contacted by the fluids. This minimizes transfer of water-to-water-sensitive formations and maintains a stable wellbore.
The invert emulsion is usually stabilized with a “primary emulsifier”, often a fatty acid or salt thereof, while the weighting material and the solids the fluid acquires during use are made oil-wet and dispersed in the fluid with a “secondary emulsifier”, typically a strong wetting agent such as a polyamide, amido-amine (partial amide of a polyamine), and the like.
Regardless of whether it is an all-oil, or an invert fluid, drilling fluids, or ‘drilling muds’ as they are sometimes called, are slurries used in the drilling of wells into the earth for the purpose of recovering hydrocarbons and other fluid materials. Drilling fluids have a number of functions, the most important of which include lubricating the drilling tool and drill pipe which carries the drilling tool, removing formation cuttings from the well, counterbalancing formation pressures to prevent the inflow of gas, oil or water from permeable rocks which may be encountered at various levels as drilling continues, and holding the cuttings in suspension in the event of a shutdown in the drilling and pumping of the drilling fluid.
For a drilling fluid to perform these functions and allow drilling to progress, the drilling fluid must stay in the borehole during the drilling operation. Frequently, undesirable formation conditions are encountered in which substantial amounts, or in some cases, practically all of the drilling fluid may be lost to the formation. Drilling fluid can leave the borehole through large or small fissures or fractures in the formation or through a highly porous rock matrix surrounding the borehole.
Most subterranean wells are drilled with the intent of forming a filter cake of varying thickness on the sides of the borehole. The primary purpose of the filter cake is to reduce the large losses of drilling fluid to the surrounding formation. Unfortunately, formation conditions are frequently encountered which may result in unacceptable losses of drilling fluid to the surrounding formation despite the type of drilling fluid employed and filter cake created.
Well drilling and servicing fluids typically contain an additive to control the loss of fluid to the formation being drilled or serviced. A variety of different substances have been used and are pumped down well bores in an attempt to reduce the large losses of drilling fluid to fractures and the like in the surrounding formations. Typical fluid loss control additives for use with oil base fluids are gilsonite, asphalt, oxidized asphalt, cellulose-based materials and various polymers, as well as almond, walnut, and other nut hulls. These fluid-loss control agents are added to the drilling or servicing fluid in an attempt to reduce the unacceptable high losses of drilling or servicing fluid to fractures and/or porous structures in the surround formation.
A number of issued patents over the years have described various polymeric compositions as fluid loss control additives in oil base muds. For example, U.S. Pat. No. 2,697,071 to Kennedy, et al. describes the use of rubber latex to regulate the viscosity and fluid loss of oil base muds.
U.S. Pat. No. 2,743,233 to Fisher describes drilling muds, and improved methods of drilling wells in the earth. Preferred embodiments of the invention reportedly relate to oil-base drilling muds having low fluid loss and increased viscosities. Another aspect of the disclosed invention pertains to oil-water emulsions used as drilling muds.
U.S. Pat. No. 4,740,319 (Patel, et al.) discloses oil base muds containing a “gelling composition” comprising a copolymer which includes 2 primary components: (1) latex type material preferably a styrene-butadiene copolymer and (2) one or more functional monomers selected from the group consisting of amides, amines, sulfonates, monocarboxylic acids, dicarboxylic acids and combinations thereof.
In U.S. Pat. No. 5,333,698, a wellbore fluid (e.g., a drilling, completion, packer, or fracturing fluid) is described that includes (a) at least one additive selected from the group consisting of emulsifiers, wetting agents, viscosifiers, weighting agents, fluid loss control agents, including polymeric fluid loss control agents, proppants for use in hydraulically fracturing subterranean formations, and particulate agents for use in forming a gravel pack; and, (b) a non-toxic white mineral oil having (i) an API gravity at 15.6° C. (60° F.) greater than 35, (ii) a content of compounds containing 14 or more carbon atoms of at least about 95 weight percent, and (iii) a pour point of at least about −30° C. (−22° F.).
U.S. Pat. No. 5,883,054 to Hernandez, et al., describes thermally stable, oil base drilling fluid systems including drilling fluid and an additive, wherein the additive includes styrene-butadiene copolymers having an average molecular weight greater than about 500,000 g/mol, and wherein the drilling fluid system exhibits fluid loss control under high temperature and high pressure conditions. According to the disclosure, the copolymers were dissolved in the base oil for 16 hours before the remainder of the additives were added.
U.S. Pat. No. 6,730,637 to Stewart, et al. describes a low toxicity drilling mud oil. In some of the described embodiments, the fluid loss characteristic of the drilling mud oil as used in a borehole can be reduced to less than 0.2 ml/30 minutes by adding about 0.05% to about 2.0% by weight of a butadiene-styrene-butadiene (BSB) block copolymer having about 20% by weight or more styrene.
The inventions disclosed and taught herein are directed to polymeric compositions and methods for the use of such compositions for reducing the fluid loss of invert oil emulsion and all oil well drilling and servicing fluids in which oil is the continuous phase.